Magnetic resistance (hereinafter, abbreviated as MR) sensors are inexpensive, small, and highly sensitive, and widely used for contactless revolution detection and position detection. The MR sensors include giant magnetoresistance (hereinafter, abbreviated as GMR) sensors, tunnel magnetoresistance (hereinafter, abbreviated as TMR) sensors, and an-isotropic magnetoresistance (hereinafter, abbreviated as AMR) sensors.
Recently, mobile devices such as cell phones and PDA (personal digital assistant) have been widespread and the mobile devices contain direction sensors using the MR sensors and may be used as navigation systems using position information by GPS (Global Positioning System). However, in adaptation of the MR sensors in the field of industrial application, high-sensitive magnetism detection technologies are not necessarily required. For example, the direction sensor detects an absolute direction with reference to geomagnetism and does not require ultrasensitive magnetism detection, and, even in encode application of revolution detection and position detection, uses a magnet as a reference signal and the ultrasensitive magnetism detection is not essential.
On the other hand, medical devices including magnetocardiograph and magnetocephalograph that detect weak and low-frequency magnetic fields generated from electrical activity of living hearts and brains (hereinafter, referred to as “biomagnetic fields”) have been recently started to be used at medical sites. For detection of the biomagnetic field, a superconducting quantum interference device (hereinafter, referred to as SQUID) is used as the ultrasensitive magnetic sensor. The SQUID is a magnetic sensor using a superconductive phenomenon and has a structure with Josephson junction. Accordingly, the SQUID requires cooling by refrigerant (liquid helium or liquid nitrogen) and is placed within a cryostat in which the refrigerant is stored. Further, a configuration that does not electromagnetically affect the Josephson junction within the SQUID is required. As described above, the SQUID is the ultrasensitive magnetic sensor, but there are problems that handling is complicated and it is impossible to make the magnetic sensor sufficiently closer to the living organism because the sensor is placed within the cryostat.
In order to measure the biomagnetic field, the sensitivity of the MR sensor at the lower frequency (100 Hz or less, particularly, 30 Hz or less) containing many biologically-originated signal components is important. The noise determining the sensitivity in the low-frequency region includes two kinds of noise of white noise and 1/f noise. These two kinds of noise is not determined only by the noise generated by the MR sensor, but determined as system noise (sensitivity) by preamplifier noise and a combination with other operation circuits.
In the report on higher sensitivity of the MR sensor described in the following NPL 2, a technique of feeding back magnetic flux to the MR sensor is disclosed. In the same literature, 1/f noise including thermal fluctuation originated from the MR sensor is reduced by the feedback technique. The technology of the literature is assumed to be used in the field of non-destructive inspection and intended to stabilize operation even in severe environments (high temperature or the like).
The following NPL 1 describes that set/reset pulse is applied to the MR sensor, magnetization of magnetoresistive elements is inverted, resulting alternating-current signals are detected, and thereby, the 1/f noise originated from the MR sensor is reduced.
In the following PTL 1, as described in paragraphs, a configuration in which “element groups in which magnetoresistance-effect elements are parallel-connected are series-connected” is disclosed “in order to obtain a magnetic field detector in which sensitivity does not vary even after adjustment of the zero-point offset voltage of output”. The configuration of PTL 1 suppresses variations in sensitivity and reduces 1/f noise originated from the MR sensor.